1. Field of the Invention
This invention relates to a method for the production of a composite pipe made of metal with its inner surface covered with a plastic layer, and to an apparatus for producing the same.
2. Description of the Prior Art
An apparatus for the production of metal pipes with their inner surfaces covered with a plastic layer has been disclosed in, for example, French Patent 2463673. This apparatus has a U-shape-forming die and a cylinder-forming die. The U-shape-forming die bends coiled metal sheets while they are conveyed in the longitudinal direction so that the edges on either side of the sheet come to face each other in a U-shaped cross-section. The cylinder-forming die forms a perfect cylinder from this metal sheet with its U-shaped cross-section and also forms a plastic layer on the inside surface of the metal sheet that has been formed into a cylinder by forcing out molten resin. This cylinder-forming die has an inner mold and an outer mold that fit together so that a cylindrical space (mold cavity) is formed. By passage of the metal sheet, which has been bent into a U-shape, through this space, the edges in the short direction across the sheet having a U-shaped cross-section overlap each other to form a cylinder. In addition, molten resin is made to flow through the inner mold, and extruded from the tip of the inner mold in the downstream side of the direction of conveyance of the metal sheet to the inner surface of the metal sheet that has been formed into a cylinder so as to cover the inner surface of the metal sheet with a plastic layer.
In order that the cylinder-forming die forms the metal sheet with a U-shaped cross-section and with its edges overlapping each other into a perfect cylinder without creation of an incompletely circular cross-section, it is preferable to extend the distance in which the U-shape of the metal sheet is formed into a cylinder by extending the interval in which the cylindrical space is formed between the inner mold and the outer mold. However, it is not possible to make the distance of the inner mold long. That is, because molten resin flows through the inside of the inner mold and is extruded from the top thereof in the downstream side to the cylinder, then if the flow path of the molten resin in the inner mold is extended, it is necessary to increase the pressure exerted on the molten resin, or it will not be possible for the molten resin to be extruded from the said tip. In particular, if the diameter of the composite pipe to be formed is small (30 mm or less), the flow path of the molten resin is greatly narrowed, and it is necessary to increase extremely the pressure applied to the molten resin.
It is necessary to place the tip on the downstream side of the inner mold, inside of which the molten resin flows, inside the metal sheet that is formed into a cylinder while the metal sheet is being conveyed, so the tip section cannot be supported. Thus, the inner mold is held on one end. If the inner mold that is held on one end is made long, there is a danger that its tip will bend. The inner mold will no longer be concentric with the outer mold that forms the metal sheet into a cylinder, so that the metal sheet cannot be formed into a complete cylinder. Moreover, when the edges of the metal sheet overlap each other to be joined by a welding method, pressure is applied to the overlapping edges, resulting in the application of pressure to the inner mold, which causes the bending of the inner mold. If the inner mold bends, the molten resin that is extruded from the tip of the inner mold will not be able to cover the inside surface of the metal sheet in the shape of a cylinder with a layer of uniform thickness. The joint in the metal sheet formed into a cylinder is ordinarily welded by an ultrasonic welding method. With this ultrasonic welding method, there is a danger that the inner mold that is being held at one end will resonate with the metal sheet that is in the shape of a cylinder. If the inner mold gives rise to resonance in the metal sheet that is in the shape of a cylinder, the joint of the metal sheet may not be welded. When a composite pipe the metal sheet of which has joints that are not satisfactorily welded is used, the inner part may not be able to resist the pressure of the fluid flowing therein and may be damaged.
The metal sheet will be formed into a cylinder while touching both the inner mold and the outer mold, and accordingly when it is made of a material softer than the materials of the inner and outer molds, there is the danger that it be damaged. If the inner surface of the metal sheet formed into a cylinder is damaged, then when it is used as a composite pipe with a high-pressure fluid flowing through the inner section, the pipe may be unable to resist the high pressure of the fluid, and may be damaged. If the area of contact of the metal sheet with the inner mold and the outer mold is a liner of synthetic resin softer than the metal sheet, damage to the metal sheet can be prevented. However, abrasion or damage can arise from the contact of the metal sheet with the liner. An abraded or damaged liner must be replaced, but in order to replace it, it is necessary to stop the conveyance of the metal sheet. Also, after replacement has occurred, it is necessary to adjust the fit of the inner mold and the outer mold so that the molds are once more concentric, and this adjustment process takes much time, which markedly lowers productivity.
When the cylinder-forming die brings together the side edges in the short direction across the metal sheet that has been bent so as to have a U-shaped cross-section, there is the possibility that the edges will push at each other and cause damage to each other, resulting in metal refuse or pieces. There is also the danger that when the overlapping is being done, the side edge of the metal sheet to be placed on the inside will be touched by the side edge that is placed on the outside, damaging the said inner surface resulting in metal refuse or pieces. Damage that arises in this way lowers the strength of the metal pipe so that when a high-pressure fluid passes through the inside, the pipe may be damaged. In order to make the pipe have a fixed inner diameter, the inner mold of the cylinder-forming die is designed so that the section in the downstream direction is made to have a larger diameter than the section upstream, and so that the larger-diameter section of the inner mold defines the inner diameter of the resulting pipe. With such a structure of the inner mold, the metal refuse or pieces created by the sliding of one edge on the other of the U-shaped metal sheet are accumulated in the step at the boundary between the large-diameter section and the small-diameter section of the inner mold. There is the chance that the accumulated metal refuse or pieces may cause new damage to the inside surface of the pipe. Also, the length in the circumferential direction of the sides overlapping with each other is short, so that if these side sections cannot be accurately bent, the side edges will not touch each other properly. In particular, one side edge on the outside bears the other side edge on the inside by touching of the interior surface of the outer side edge, so it is possible that a spring action will cause the inner-side edge to spring the outer-side edge upward. This springing upward of the outer-side edge causes damage to the outer mold of the cylinder-forming die and accelerates abrasion of the said mold. If strong friction between the side edge of the metal sheet and the outer mold is generated, it may become difficult to convey the metal sheet by pulling. If friction becomes still greater, the metal sheet conveyed will be broken. At the time of overlapping, there is the danger that the overlapping of the edge on the outside and the edge on the inside may be reversed; in this case, it is not possible to carry out the succeeding process of welding with accuracy. There is also the chance that the edge of the inner side will fold back the edge of the outer side, and in these circumstances, the composite pipe obtained will not have the strength to resist the flow of a high-pressure liquid.